COLOMBO (Reuters) - Sri Lanka will hold a presidential election in January, nearly two years ahead of schedule, the government's spokesman said on Monday, with Mahinda Rajapaksa expected to run for a third six-year term.

The announcement came amid signs Rajapaksa's popularity is fading after accusations that his party is abusing power.

"There will be presidential elections in January," spokesman Keheliya Rambukwella told a public gathering in the central city of Kandy. He declined to name an exact date.

The move may affect Pope Francis's planned visit to Sri Lanka from Jan. 13-15, because the Vatican may not want it to go ahead so close to an election.

According to the constitution, President Rajapaksa's second six-year term ends in November 2016. However, legal experts backing him say he can call for an early election after four years in office.

Rajapaksa, 68, came to power in 2005 and retained the presidency in 2010 on a wave of popularity after the military defeat of Tamil Tiger separatists in 2009, ending a 26-year civil war.

Rajapaksa has been accused of rights abuses and nepotism. He insists any relatives in parliament are there because people elected them and not because he chose them.

Critics says he enjoys undue powers under a system known as "executive presidency" introduced by a 1978 constitution.

Rajapaksa said during election campaigns in 2005 and 2010 he would abolish the executive presidency, but has not done so yet, saying it is still needed to curb any potential threat from the defeated Tamil Tigers.

"I will be ready to abolish the executive presidency, if those who are seeking separatism abandon their demand," Rajapaksa told minority Tamils on Oct. 12 in the northern town of Kilinochchi, the former de-facto rebel capital.

UNLIMITED TERMS

The 2009 victory over the separatists helped him to secure more than two thirds of parliamentary seats, enabling him to change the constitution that had limited leaders to two terms in office.

The public euphoria has long faded, however. The ruling United People's Freedom Alliance party won a local election in the southeastern province of Uva last month, but with 21 percent less support than in 2009.

There is controversy over whether Rajapaksa is legally entitled to run for a third term. Some experts say he cannot do so without winning a further parliamentary vote to make the constitutional amendment retrospective in his own case, along with a referendum.

"If he wants to make the law retrospective he needs to do it through proper process. He needs a two-thirds majority in the parliament to pass it and needs a referendum," Upul Jayasuriya, the president of the Sri Lanka Bar Association, told Reuters.

Lawyers backing Rajapaksa say the Supreme Court alone can interpret the constitution and determine whether he can run.

Opposition parties as well as one of his own coalition partners have said a third term for Rajapaksa without abolishing the executive presidency would be illegal.

Coalition partner Jathika Hela Urumaya, a Buddhist extremist party which helped him to win majority Sinhala-Buddhist votes in the last two elections, has called on him to abolish the executive presidency and restore democracy.

No rival with a realistic chances of defeating Rajapaksa has so far emerged.

Criticism from abroad of Sri Lanka's conduct during the final phase of the war has boosted Rajapaksa's popularity, as many ethnic majority Sinhalese consider such accusations unfair.

The United Nations estimated in a 2011 report that about 40,000 Tamil civilians were killed in the final weeks of the conflict, mostly by the army. Sri Lanka has rejected the allegation.